Various types of fluid filters are in wide use. Some examples of fluid filters include filters that are used in connection with engines and/or vehicles, such as, for example, engine oil and/or transmission fluid filters. Many typical transmission filters include a housing having an inlet and an outlet, with the housing sometimes having a relatively flat thin profile in order to fit in the sump of the transmission. Transmission fluid often enters these filters through an inlet port at the bottom flat side, and exits through an outlet port on the top side. The housing typically has two halves, a top half and a bottom half, that are attached to each other at their periphery, either by crimping or welding. The housing halves may both be plastic, or one or both of the halves may be metal.
One example of such type of filters is a single-layer flat sheet media filter, in which a flat sheet of filtering media spans across the inside of the housing, and for example, can be held at the edges of the housing near the attachment of the two housings. Another type of such filter is a so-called bag filter, in which a sheet of filtering media is folded over and sealed to itself around its entire non-folded periphery, and thus is formed essentially into an enclosed bag, with a hole forming an opening into the bag. The opening is attached to the inlet port, typically by being sealed to the inlet port. In such bag filters, the fluid flows directly from the inlet port of the housing into the inside of the bag, and passes through the bag media until it is outside the bag and then flows out the outlet port of the housing.
The media for the flat sheet or bag can be of a very coarse filtration type, such as a metal or plastic screen, or coarse fabric, or can range to a finer type such as, for example, a medium or very fine woven or felt type on fine fabric material.
These filters have been found to be very suitable in use, for example in vehicle transmissions to filter vehicle transmission fluid. However, a compromise must often be reached with regard to the degree of fineness of filtering media. For example, the fluid or oil is typically more viscous or thick in a cold start-up condition, and then becomes more thin, or less viscous, as the transmission heats up to its steady operating temperature. A media that is suitable for filtering at the higher operating temperature may be so fine that fluid is impeded or blocked to an undesirable extent during cold start-up operation. On the other hand, a media that is coarse enough to provide some filtering at start-up operation, as well as the necessary flow, may not provide desirably fine filtration at any temperature.
One approach that has been used to overcome this problem has been to attempt some form of bypass, whereby the fine filter material is partially or fully bypassed during cold start operations. However, it is always desirable to have new and improved filter arrangements.